The invention relates to regulating the maximum output current of a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically A coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
The fuel cell stack may be part of a fuel cell system that supplies power to a load, such as house, for example. In this manner, among its various components, the fuel cell system may include an inverter to convert the DC voltage that furnished by the stack into AC voltages that may be furnished to the house. The fuel cell system may also include a reformer to convert a hydrocarbon (natural gas or propane, as examples) into a hydrogen gas flow. The hydrogen gas flow needs to be large enough to satisfy the stoichiometry dictated by the above-described equation. Therefore, higher current levels require larger flow rates and thus, require more hydrogen production by the reformer.
The fuel cell system typically monitors the output power of the system and regulates the production of the reformer based on the monitored power. Thus, an increased power demand from the house typically requires an increase in the production by the reformer. A conventional reformer may have a relatively slow transient response that causes any increase in production to significantly lag the increased demand for power. As a result, when the power that is demanded by the house suddenly increases, the fuel cell stack may xe2x80x9cstarve xe2x80x9d due to the lack of a sufficient hydrogen flow until the production of hydrogen by the reformer increases to the appropriate level. This fuel starvation, in turn, may damage fuel cells of the stack.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a method includes establishing a fuel flow through a fuel cell stack to produce a current. The cell voltages of the fuel cell stack are scanned to determine the minimum cell voltage. The current is limited to a maximum current limit that is based on the minimum cell voltage.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.